For longer events I use a Topeak Mondopack XL (a large under saddle pack, about 2l, I think) with a small Decathlon handlebar bag (the same one in the photo at the top of this page, I believe).
I fitted a KlikFix bracket to the handlebar bag instead of using the original velcro attachments. That keeps the bag clear of my dynamo light and the top horizontal, so I can open it to grab snacks, etc on the move, without anything falling out.
The Decathlon bag is very neat and I don't carry anything heavy, so it doesn't affect the handling, and I was surprised to find it actually seems to be quite aerodynamic. I have out-freewheeled another rider on tri bars!
That set up did me fine for LEL, and I could grab everything quickly without rooting around in roll top bags.